The present invention relates to amplifiers such as those employed in AM broadcasting and, more particularly, to such amplifiers including mean for minimizing distortion and noise resulting from variations in the DC power supply.
It is known that noise and/or other variations appearing on the output of a DC power supply connected to the input of an amplifier may result in distortion. The power supply noise is of particular concern in high power amplifiers, such as those found in transmitters conventionally used in commercial AM broadcasting. It is known to employ large capacitors connected across the power supply to reduce low frequency AC supply voltage variations caused by power supply ripple and low frequency transmitter modulation. These power supply variations may well be reduced substantially by connecting a large amount of capacitance across the power supply output. However, such capacitors employed with high power DC power sources are quite large both in volume and in weight. The inclusion of such capacitors is undesirable from the standpoint of cost, size and weight.
It is also known that some AM transmitter circuits employ a low frequency distortion correction circuit that takes a sample of the power supply output and then generates a correction signal that is used to compensate for power supply sag. Such prior art circuits include, for example, the U.S. Pat. Nos. to H. I. Swanson et al., 4,731,731, H. I. Swanson, 4,580,111, and D. H. Covill, 4,605,910. Each of these patents discloses circuitry for minimizing modulation distortion of an amplitude modulated RF carrier signal resulting from variations in the DC power supply. In each case, a feed forward technique is employed in which a sample of the input DC voltage signal is obtained and is combined with the input audio signal to compensate for variations in the magnitude of the DC supply voltage prior to supplying the input audio signal to the amplification stages of the transmitter.
In the prior art discussed immediately above, the AC and DC components of the power supply sample are used for both long term DC supply changes and short term sags such as power supply ripple or low frequency transmitter modulation. In low power transmitters, correction loops typically treated the AC and DC components equally in terms of phase and gain of the signals, since large amounts of supply filter capacitance were practical in most cases. However, with higher power transmitters, such as those exceeding 50 kilowatts, the cost of such filter capacitance prohibits their use for minimizing power supply sag. Moreover, at higher power levels, the transmitters tax the AC main input lines and may possibly cause additional power supply sag with modulation. In view of these factors, it has been found desirable that the AC component of the power supply sample be separated from the DC component so that additional gain and phase correction may be made to the AC component independent of variations in the DC component to achieve optimum low frequency distortion and minimum AM noise.